It is typically desirable for a map to reflect the current conditions of constantly changing roads, facilities and so on accurately. Therefore, in a known system employed in a map database storing map data in the form of electronic data for use in a navigation device or the like, for example, update data are provided appropriately via a communication network, a data recording medium, or the like, and the map database is updated using the update data. An updating system such as the following is described in Japanese Patent Application Publication 2007-328739 (pages 17 to 21, FIG. 1) as an example of this type of map database updating system.
This updating system is constituted by a terminal device such as a navigation device which is operated in accordance with a predetermined operating program, and a server device that provides a difference data file as update data. The terminal device includes a local storage database storing map data in an update data format for performing a difference update in accordance with the difference data file, and a reference database storing map data in a reference data format for reference by the operating program. Upon reception of the difference data file, the terminal device performs a difference update on the map data stored in the local storage database in the update data format, and then performs processing to convert the updated map data into map data in the reference data format and store the converted data in the reference database. Thus, the map data in the reference database that is referenced by the operating program are updated.
In the map database updating system described above, a large amount of file access is performed on the map databases including the local storage database to read required data when a difference update is performed on the map data in the local storage database in accordance with the difference data file, when the updated map data in the local storage database are converted into map data in the reference data format, and so on. Therefore, the time required for this file access occupies a large proportion of the time required for the map database update processing. One effective solution for reducing the file access time is to store read data in a cache area of a memory and then read the same data from the cache area as required.
However, in cases where a large-capacity memory cannot be installed in the terminal device due to cost or size limitations and so on, it may be difficult to secure a cache area having a sufficiently large capacity. In a terminal device such as a navigation device installed in a vehicle, size limitations and so on are particularly severe in comparison with a stationary terminal device such as a personal computer, and it is therefore difficult to install a large-capacity memory. Accordingly, a large-capacity cache region cannot easily be secured in the memory. As a result, a large amount of data cannot be stored in the cache area and the proportion of the cache used for file access cannot be increased, making it impossible to achieve a large reduction in the file access time.